I have no reassurance to offer, sorry, except that he'll become more deaf as he ages and then it gets better.
Some people use Rescue Remedy or veterinary sedatives when they know there're going to be loud noises - Nov 5th etc - but that doesn't help with the unexpected, I know.
A safe bolt-hole he can always get to is a must for him, poor boy.
I have always assumed that working collies are pretty much unconsciously bred for good hearing - a good collie needs to be able to hear directions on the hill, and that command is often by whistle, so is high-pitched. It's the good workers that get bred from, so the hearing gets better and better over the generations.
I have had several collies react to the military jet engines, and the moorland farm was a key waypoint for the RAF so we'd get jets flying over and turning over us pretty frequently. Ted used to serenade them with his lovely collie song...
Collies around here have to be robust about bangs; we've a military base nearby does explosions regularly - they literally rock the houses, never mind collies' ears.